In some conventional bipolar signal translators, a pair of interconnected element stacks are used in performing voltage translation. S. Platt et al disclose such a translator for converting an input voltage signal into an output voltage signal at a different voltage level in U.S. Pat. No. 3,788,640. The element stacks in this single-input device are connected to each other through a pair of lower NPN transistors configured as a current mirror. The base of a first of the lower transistors is connected directly to its collector and to the base of the second lower transistor. The emitters of both lower transistors are tied directly to a low voltage supply.
In one version of Platt et al, the collector of the first lower transistor is coupled through a first resistor to the emitter of an upper NPN reference transistor whose base receives a fixed reference voltage. The collector of the second lower transistor is coupled through a second resistor to the emitter of an upper NPN input transistor whose base receives the input voltage. The collectors of both upper transistors are tied directly to a high voltage supply. The collector of the second lower transistor also drives the base of an NPN output transistor whose collector is coupled through a pull-up output resistor to the high voltage supply and whose emitter is tied to the low voltage supply. The collector of the output transistor provides the output voltage.
All of the transistors in Platt et al are conductive except for the output transistor which normally switches on and off. When the input voltage equals the reference voltage, the voltage at the base of the output transistor is at a selected nominal value below the input voltage. Because of the current mirror, the currents through the element stacks are virtually equal. When the input voltage changes, the base voltage of the output transistor changes by approximately the same amount and in the same direction. This causes the output transistor either to turn on and saturate if the input voltage change is sufficiently positive or to turn off if the input change is sufficiently negative.
In another version of Platt et al, the reference and input voltages are reversed--i.e., the input voltage is supplied to the upper transistor which previously received the reference voltage, and vice versa. As a result, the currents through the element stacks are variable, though still substantially equal. A change in the input voltage above or below the reference voltage causes the base voltage of the output transistor to change by the same amount but in the opposite direction. The output transistor then turns on if the input voltage change is sufficiently positive, and vice versa.
The translators of Platt et al appear capable of generally providing good voltage translation. However, noise at the low voltage supply can cause instability because only the base-emitter junctions of the lower transistors are present to receive the noise. In addition, power is wasted since only the output resistor lies between the high voltage supply and the output point.
In U.S. Pat. No. 3,986,045, R. Lutz discloses an emitter-coupled logic-to-transistor-transistor logic (ECL-to-TTL) translator configured similarly to Platt et al. Input signals of opposite logical polarity are supplied to the upper transistors to make the Lutz device a differential-input translator. In addition to this, there are several configurational differences. The base of the first lower transistor is coupled to its collector by way of the base-emitter junction of an NPN feedback transistor. This feedback coupling is undesirable in some applications since it might cause oscillations in the feedback loop. Resistors in the emitters of the lower transistors are employed to improve stability. The upper end of the output resistor is connected to the transistor emitter connected to the first resistor so as to enable the output transistor to saturate more rapidly. This connection can, however, create difficulty in the current matching between the lower transistors.
W. Wilhelm discloses another similar differential-input translator in published European Patent Application No. 009083 in which the lower transistors are again arranged as a current mirror with their emitters tied directly to the low voltage supply as in Platt et al. The Wilhelm device thus has basically the same deficiencies as Platt et al.